Cable television systems, or CATV systems, are used in a widespread manner for the transmission and distribution of television signals to end users, or subscribers. In general, CATV systems comprise a transmission subsystem and a distribution subsystem, The transmission subsystem obtains television signals associated with a plurality of CATV channels and generates a broadband CATV signal therefrom. The distribution subsystem then delivers the CATV broadband signal to television receivers located within the residences and business establishments of subscribers.
One problem facing CATV service providers is signal leakage. Signal leakage refers to the transmission and/or reception of signals through breaches or other nonconformities in the CATV distribution subsystem. In particular, the distribution subsystem, which typically comprises coaxial cable, amplifiers and other devices, ideally provides a relatively low-loss conduit between the CATV transmission subsystem and subscriber' television receivers. If, however, portions of the distribution subsystem are physically damaged, for example, the coaxial cable is damaged, kinked or broken, then CATV signals may leak through the damaged portions, causing unwanted transmission into the atmosphere.
The primary problem associated with the transmission of the CATV signal into the atmosphere via leakage is potential interference with aeronautical communications. Portions of the allocated CATV bandwidth overlap with frequencies allocated for aeronautical communication. Excessive leakage of CATV signals can therefore undesirably interfere with aeronautical-related signal transmission and reception.
Another problem caused by excessive leakage arises in the context of reverse path communication. Reverse path communication refers to communication signals generated by CATV subscribers and transmitted to the CATV transmission subsystem. The problem caused by leakage is that signal ingress due to leakage can undesirably interfere with such reverse path communications. Signal ingress is the infiltration of spurious external signals into the CATV distribution subsystem. If there are several leakage points in the distribution subsystem, then the spurious signals from those several leakage points in the CATV distribution subsystem will tend to accumulate at the CATV transmission subsystem. The accumulated spurious signals can have considerable energy, thereby potentially causing interference with reverse path communication signals.
As a result, signal leakage is an undesirable phenomenon that CATV service providers strive to reduce.
In order to reduce leakage, CATV service providers must first determine the location of leakage points in the distribution subsystem. Various leakage detection devices are currently available that assist in the location of leakage points. Such devices typically include an antenna and a receiver that is tuned to a particular frequency in the CATV signal bandwidth. The detector further includes a signal strength measurement circuit. To detect leakage, a technician typically drives along a route that traces a portion of the CATV distribution system, preferably in the vicinity of a suspected leakage location. If the signal strength measurement circuit detects a relatively large amplitude signal at a particular location, then a leak may be indicated in or near that location. The technician may then use the leakage detector to pinpoint the source of the leak. Once the source of the leak is pinpointed, corrective action may be taken.
A drawback of the above described leakage detection devices is their inability to distinguish CATV signals leaked from the system under test from other signals in the same bandwidth. This drawback is becoming of increasing importance due to the proliferation of CATV service providers. In particular, two or more CATV service providers often have portions of their distribution systems that overlap, or at least are disposed in close proximity to one another. As a result, when a technician detects leakage in a particular location, that leakage may either be caused by the system under test or by another system. Because CATV service providers are primarily interested only in leakage in their own distribution system, it is desirable to ascertain the identity of the source of the leak.
One prior art method of addressing the problem of differentiating the leakage signals from a system under test from other signals is described in U.S. Pat. No. 4,237,486 to Shimp, issued Dec. 2, 1980. Shimp describes a method of modulating a distinctive tagging signal on an unused CATV channel frequency at the transmission subsystem. The leakage detector is then tuned to that CATV channel frequency and used for leakage detection. If a relatively large signal strength is detected, the leakage detector then attempts to isolate or detect the distinctive tagging signal. If the tagging signal cannot be detected, then it is determined that the detected signals are not caused by leakage in the system under test. If, however, the tagging signal is detected, then it is determined that the detected signal is due to leakage in the system under test.
While the Shimp patent discusses a method of determining whether detected signals in the bandwidth of interest are caused by leakage by the system under test, that method requires an unused channel frequency. The requirement that an unused frequency be used undesirably consumes valuable CATV bandwidth.
U.S. Pat. No. 5,608,428 to Bush proposes a system in which a tagging signal is modulated onto an active video carrier, in other words, an in-use channel frequency. Specifically, a low frequency oscillating signal is modulated onto the active video carrier. That low frequency oscillating signal may then be detected by a leakage detector to determine whether a detected signal is caused by leakage by the system under test. The low frequency signal is chosen such that automatic gain control ("AGC") circuit in many subscriber' television receivers will reduce the interference caused by the low frequency signal. While the system proposed by Bush does not require an out of service channel for tagged leakage detection, the modulation of the low frequency signal nevertheless can cause undesirable signal distortion to some subscribers.
Accordingly, there is a need for a leakage tagging method and apparatus that does not require an out of use channel, but also has a reduced likelihood of producing distortion in the used channels of a CATV signal.